SONET, or Synchronous Optical NETwork, is now the preferred standard for optical transport of telecommunications traffic in North America. This standard has been developed and implemented over the last decade to give telecommunications carriers important benefits that are difficult to achieve using previously available asynchronous transport technology. Among the most significant of these advantages are: greater compatibility among equipment from different manufacturers; synchronous networking for improved reference timing of network elements; enhanced operations, administration and provisioning capabilities; and compatibility with any service mix including both traditional services and newer services such as Asynchronous Transfer Mode (ATM) traffic.
The SONET hierarchy is built upon a basic signal of 51.84 megabits per second (Mbps), known in the art as a level-1 synchronous transport signal frame, denoted STS-1 and sometimes referred to as a “time slot”. A byte-interleaved multiplexing scheme can be applied to multiple STS-1 frames, resulting in a digital signal having a rate of N times the basic rate, where N is typically 1, 3, 12, 48 or 192. The optical form of an STS signal is called an optical carrier (OC) and thus an STS-N signal and an OC-N signal have the same rate.
The STS-1 frame has a portion of its capacity used for delivering payload, while the remaining portion is devoted to overhead. The payload refers to the data or traffic part of a signal, while the overhead consists of signalling and protocol information. The use of the SONET overhead allows communication between intelligent nodes in the network, which enables administration, surveillance, provisioning and control of the network to be carried out from a central location.
Today's SONET transport networks typically employ a number of different topologies to satisfy important objectives such as network simplicity, cost containment, bandwidth efficiency and survivability. For instance, an optical hubbing configuration may be used to eliminate the need for a costly and complicated arrangement consisting of several back-to-back network elements. Another example is the deployment of self-healing rings to assure survivability of traffic around the ring through the provision of redundant communications paths.
By way of example, a two-fiber bidirectional line-switched ring (2F BLSR) is a survivable SONET transport architecture that protects against cable cuts and node failures by providing duplicate, geographically diverse paths for each service to be delivered. In a 2F BLSR, the two fibers carry unidirectional traffic in opposite directions and the bandwidth of each unidirectional fiber is split between working traffic and protection traffic.
A service path is provisioned through a 2F BLSR by selecting endpoint network elements and one or more STS-1 time slots linking the service entry and exit points. Although two communication paths are available around the ring, a service reaches its destination by travelling along the working path of only one of these. Intermediate nodes on the service path, if they exist, simply pass the service from east to west (or vice versa) without modifying the STS-1 channel assignment.
In the event of a failure or degradation of an optical span, the automatic ring protection switching functionality of SONET reroutes affected traffic away from the fault within 50 milliseconds in order to prevent a service outage. Traffic is redirected by looping back STS-1 time slots across the protection path in the other direction. The normally unused protection bandwidth thus forms a logical bridge over the defective span, thereby maintaining service for all terminating and pass-through traffic.
Another characteristic of the 2F BLSR architecture is that it allows individual STS-1 channels to be reused as traffic is terminated at various locations around the ring. This feature makes the 2F BLSR architecture ideally suited to the mesh and node-to-adjacent-node traffic patterns found in interoffice networks and also in certain types of access networks. The reuse of STS-1 channels also offers important bandwidth synergies in ATM networks.
In a four-fiber (4F) BLSR, two pairs of unidirectional fibers link adjacent nodes in the ring. One fiber pair exclusively carries working traffic while the other pair serves as a protection facility. If a fault affects a working fiber along a span, traffic is rerouted along the corresponding protection fiber. If the fault affects both the working and protection fibers, automatic ring protection switching redirects traffic in a manner similar to a 2F BLSR. However, instead of looping back time slots within the same fiber pair as in a 2F BLSR, traffic is transferred from the working pair to the protection pair.
In many cases, it is desirable to exchange information not only between nodes in a ring but also between nodes located in separate rings. Using SONET, adjacent rings are easily connected to one another by virtue of arranging one or more nodes from each ring to communicate as gateway nodes. While the above-described route diversity fully protects all working traffic passing from node to node along an individual ring, service paths must nevertheless be protected on an end-to-end basis. This means that survivable inter-ring connections are required as traffic passes through the designated gateway nodes from the ring serving the entry point to the ring serving the termination point.
To this end, protection of inter-ring traffic can be provided by the SONET “matched nodes” configuration, in which redundant (i.e., duplicate) routing is provided across inter-ring boundaries. For example, FIG. 1 illustrates a matched nodes configuration as applicable to two 2F BLSR rings wishing to communicate with one another. Node 6 in ring 2 and “matched” node 14 in ring 8 have been chosen as primary gateway nodes, while node 10 in ring 2 and “matched” node 16 in ring 8 have been configured as secondary gateway nodes.
Within primary gateway node 6, a drop-and-continue router 4 is used for duplicating a working signal and forwarding copies of the signal to secondary gateway node 14 in ring 8 as well as to secondary gateway node 10 within ring 2. Secondary gateway node 10 is equipped with means for forwarding the received copy to secondary gateway node 16 in ring 8, which then sends the copy to primary gateway node 14. Thus, under normal operating conditions, primary gateway node 14 in ring 8 receives two copies of the signal transmitted by primary gateway node 6 in ring 2. In FIG. 1, the duplicate paths between the two rings are shown as a thick solid line.
Of course, both copies of the delivered service are not required at the primary gateway node 14 in ring 8. For this reason, node 14 is equipped with a service selector 12 that chooses one of the copies as a function of signal integrity, which can be inferred from standard parameters such as the line and/or path alarm indication signal (AIS). For the explanatory purposes, it is assumed that the service selector 12 is programmed to select the “primary” inter-ring signal arriving directly from node 6 rather than the “secondary” inter-ring signal arriving via node 16.
In the event of a failure within either ring (e.g., a fiber fault along lines B–B′ or C–C′ in FIG. 1), the matched nodes configuration provides no significant benefit, as the automatic ring protection switching facility of SONET will cause the working signals to be looped back over the appropriate protection path in the respective ring. Rather, the classical advantage of the matched nodes configuration is that inter-ring traffic is protected in the event of a failure on the inter-ring span between primary gateway nodes 6 and 14, e.g., a fiber fault along lines A–A′ in FIG. 1.
In the latter case, node 14 notices that the primary inter-ring signal arriving from node 6 is lost and switches its service selector 12 to the secondary inter-ring signal arriving from node 16. Although the primary inter-ring signal was lost, the secondary inter-ring signal remains unaffected by the fiber fault. For this reason, the matched nodes configuration is often used for providing survivable connections between rings.
However, the matched nodes configuration is afflicted with several serious drawbacks which make it a rather unsatisfactory choice for ensuring the protection of inter-ring traffic. Firstly, duplication of the data signal by the drop-and-continue router 4 within primary gateway node 6 results in a waste of bandwidth under non-fault operating conditions. If other network elements were connected between nodes 6 and 10, for example, then the bandwidth available to carry traffic destined for (or originating from) such intermediate nodes would be notably reduced relative to the working bandwidth available in the absence of matched nodes.
A second problem arises due to the reliance of the service selector 12 within node 14 upon the integrity of the primary inter-ring signal received from node 6. Specifically, it is noted that at least two distinct scenarios may result in loss of the primary inter-ring signal. One of these is a fiber fault along lines A–A′ in FIG. 1, which clearly necessitates a quick reaction by the service selector 12 in order to avoid a prolonged outage. In a second instance, the primary inter-ring signal can also be lost due to a failure on ring 2 along lines B–B′. However, the standard automatic ring protection switching functionality inherent to the design of the ring 2 will cause the “lost” signal to be looped back around the protection path of ring 2 and delivered, uncorrupted, to node 6 via node 10.
Therefore, in order to avoid prematurely switching the service selector 12, it is necessary for the secondary gateway node 14 to pause upon initial detection of a loss of the primary inter-ring signal in order to determine whether the signal loss is indeed irreparable. The length of this pause, known as the “hold off” time for matched nodes, has been specified to be 100 milliseconds in Bellcore standard GR-1230, issue 3, chapter 7, hereby incorporated by reference herein. Clearly, if the signal loss truly is due to a fault along line A–A′, then the 100 millisecond hold off time disadvantageously results in an unnecessary outage, with consequences ranging from mild to severe for the telecommunications service provider.
A third drawback of matched nodes becomes apparent when considering the connection of a large number of lower-capacity rings to a common main ring. When using the matched nodes configuration, each additional ring requires two of its nodes to be equipped with matched nodes functionality. Depending on the number of fibers and the capacity of each ring, the number of additional rings can be quite large (up to 32 2F OC-12 rings or up to 8 OC-48 rings connected to a single 4F OC-192 BLSR), leading to an exorbitant number of network elements having to be equipped with matched nodes capability.
The background information provided above clearly indicates that there exists a need in the industry to provide a method and apparatus for enabling the survivability of inter-ring traffic which is faster, more bandwidth efficient and better integrated than the solutions currently applied in the industry.